Thermal actuators

ABSTRACT

A thermal bend actuator ( 6 ) is provided with a group of upper elongate actuator arms ( 23, 25, 26 ) and a group of lower elongate actuator arms ( 27, 28 ) which are non planar, so increasing the stiffness of the arms. The arms ( 23, 25, 26, 27, 28 ) may be spaced transversely of each other and do not overly each other in plan view, so enabling all arms to be formed by depositing a single layer of arm forming material.

CROSS REFERENCE TO RELATED APPLICATION

This is a Continuation Application of U.S. application Ser. No.10/258,518 filed on Oct. 25, 2002, which is a 371 of InternationalApplication Serial No. PCT/AU01/00501 filed on May 2, 2001 the entirecontents of which are herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the field of micro electromechanicaldevices such as ink jet printers. The present invention will bedescribed herein with reference to Micro Electro Mechanical Inkjettechnology. However, it will be appreciated that the invention does havebroader applications to other micro electromechanical devices, e.g.micro electromechanical pumps or micro electromechanical movers.

BACKGROUND OF THE INVENTION

Micro electromechanical devices are becoming increasingly popular andnormally involve the creation of devices on the μm (micron) scaleutilising semi-conductor fabrication techniques. For a recent review onmicro-mechanical devices, reference is made to the article “The BroadSweep of Integrated Micro Systems” by S. Tom Picraux and Paul J.McWhorter published December 1998 in IEEE Spectrum at pages 24 to 33.

One form of micro electromechanical devices in popular use are ink jetprinting devices in which ink is ejected from an ink ejection nozzlechamber. Many forms of ink jet devices are known.

Many different techniques on ink jet printing and associated deviceshave been invented. For a survey of the field, reference is made to anarticle by J Moore, “Non-Impact Printing: Introduction and HistoricalPerspective”, Output Hard Copy Devices, Editors R Dubeck and S Sherr,pages 207-220 (1988).

Recently, a new form of ink jet printing has been developed by thepresent applicant, which is referred to as Micro Electro MechanicalInkjet (MEMJET) technology. In one form of the MEMJET technology, ink isejected from an ink ejection nozzle chamber utilising an electromechanical actuator connected to a paddle or plunger which moves towardsthe ejection nozzle of the chamber for ejection of drops of ink from theejection nozzle chamber.

In previous designs of actuator a lower arm is deposited as a generallyplanar single layer, a sacrificial spacing layer is formed and then anupper arm is deposited as a generally planar layer.

A major portion of the cost of a manufactured wing semiconductormanufacturing techniques device depends on the number of separate layersrequired to be deposited during fabrication. Reducing the number ofseparate layers that need to be deposited reduces the cost of thedevice.

The efficiency of the thermal actuator is roughly inversely proportionalto the mass of the actuator material. The actuator arms need to have acertain stiffness. If the stiffness of the arms can be maintained whilstdecreasing mass, the efficiency of the actuator can be improved.

The present invention, in preferred forms, aims to address either orboth of these issues.

SUMMARY OF THE INVENTION

In one aspect of the invention there is provided a thermal actuatorwhich may be manufactured with material for upper and lower arm setsbeing deposited in a single step. This is achieved by having the armsarranged in a non-overlapping manner.

Accordingly, in one aspect the invention provides a thermal actuator formicro mechanical or micro electromechanical devices, the actuatorincluding:

-   -   a supporting substrate,    -   an actuator extension portion,    -   a plurality of actuator arms comprising at least one first arm        and at least one second arm spaced apart from the first arm,    -   the at least one first arm attached at a first end thereof to        the substrate and at a second end to the extension portion, the        first arm being arranged, in use, to be conductively heatable,    -   at least one second arm attached at a first end to the        supporting substrate and at a second end to the extension        portion,    -   the first arm being arranged, in use, to undergo thermal        expansion, thereby causing the actuator to apply a force to the        extension portion, and    -   wherein, in plan view, said plurality of actuator arms do not        overlap eachother.

In a second aspect of the invention there is provided a thermal actuatorhaving at least one of the upper and lower arms including stiffeningmeans.

Accordingly a second aspect of the invention provides a thermal actuatorfor micro mechanical or micro electromechanical devices, the actuatorincluding:

-   -   a supporting substrate,    -   an actuator extension portion,    -   at least one first arm attached at a first end thereof to the        substrate and at a second end to the extension portion, the        first arm being arranged, in use, to be conductively heatable,    -   at least one second arm attached at a first end to the        supporting substrate and at a second end to the extension        portion, the second arm being spaced apart from the first arm,    -   the first arm being arranged, in use, to undergo thermal        expansion, thereby causing the actuator to apply a force to the        extension portion, and    -   wherein, in transverse cross-section, at least one of the first        arm and the second arm are non-planar in order to increase the        bending resistance.

In transverse cross-section both the first and second arms may benon-planar.

The actuator preferably includes two first arms electricallyinterconnected at the second end and preferably includes three secondarms. However, other combinations of upper and lower arms are within thescope of the invention.

The at least one first arm and at least one second arm may be spacedtransversely relative to each other and preferably in plan view do notoverlap.

The edges of the first and second arms may be located in a common plane.

Where edges of the first and second arms are located in a common planethe first arm preferably has a centre of inertia located to one side ofthe common plane and the second arm preferably has a centre of inertialocated to the other side of the common plane.

In transverse cross-section the at least one first arm and/or the atleast one second arm may include an edge portion extending away from theother arm. The edge portion may extend inwardly.

In transverse cross-section the at least one first arm or the at leastone second arm, or both, may have a U, V, C or W profile.

In a third aspect of the invention there is provided a method ofmanufacturing a thermal actuator in which material for upper and lowerarms is deposited in a single step.

Accordingly a third aspect of the invention provides a method of forminga thermal actuator including the steps of:

-   -   a) depositing a first layer of sacrificial material;    -   b) depositing at least a second layer of sacrificial material on        a selected part or parts of the first layer;    -   c) depositing an actuator forming layer of material over the        first and second layers of sacrificial layers; and    -   d) selectively removing portions of the actuator forming        material to form at least one first arm deposited on the first        layer and at least one second arm deposited on at least part of        the second layer.

Step b) may include depositing one or more additional layers ofsacrificial material on selected parts of the second layer.

The additional layer or layers may be deposited on only part of thesecond layer.

The at least one first arm so formed may be planar or non planar. Intransverse cross-section the at least one first arm or the at least onesecond arm, or both, may have a U, V, C or W profile.

The layers may be deposited so that, when formed, the transverse edgesof the at least one first arm lie in a first plane and the transverseedges of the at least one second arm lie in a second plane. The firstand second planes may lie in a common plane.

In plan view the at least one first arm and the at least one second armmay overlap or may be spaced transversely to each other.

In plan view when there are at least two first or second arms the armsalternate.

In the preferred embodiments, an improved thermal bend actuator isformed through the depositing and etching of a number of layers with theactuator arms being in the form of a corrugated or non planar structureso as to provide for increased stiffness in the bending direction. Thecorrugated titanium nitride layer of the thermal bend actuator isdefined utilising standard lithographic steps and in a number ofembodiments is of such a nature that the masking edges occur on the sameplanar layer such that all edges will be simultaneously within the depthof field of a stepper process.

It is important that the arms of an actuator have sufficient stiffnessto avoid buckling. For a given stiffness the efficiency of the thermalactuator is roughly inversely proportional to the mass of the actuatormaterial. The utilization of a corrugated or non-planar structureprovides for improved stiffness characteristics for a given thermalmass. The corrugations achieve a higher stiffness without increasing thethickness and thereby increasing the mass of the actuator. This providesan overall increase in efficiency. Generally, simulations indicateefficiency increases of around 50% are possible.

In the preferred embodiment, a thermal bend actuator is formed by thedeposition of a single layer of titanium nitride so as to formspaced-apart actuator arms which do not coincide in a vertical plane.The advantage of utilising a single layer of out of plane actuator isthat the number of layers to be deposited and patterned in the structureare substantially reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms which may fall within the scope of thepresent invention, preferred forms of the invention will now bedescribed, by way of example only, with reference to the accompanyingdrawings, in which:

FIG. 1 shows, in schematic form, the operation of a thermal bendactuator ink jet printing device;

FIG. 2 shows a plan view of thermal bend actuator of a first embodimentof the invention;

FIG. 3 shows a cross-section of a thermal bend actuator of the firstembodiment of the invention;

FIG. 4 shows a cross-section of a thermal bend actuator of a secondembodiment of the invention;

FIG. 5 shows a cross-section of a thermal bend actuator according to thefirst embodiment partway through the manufacturing process;

FIG. 6 shows a cross-section of a thermal bend actuator according to thesecond embodiment partway through the manufacturing process; and

FIG. 7 illustrates a sectional view through a portion of a nozzlechamber which is formed simultaneously with the formation of the thermalbend actuator.

DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS

The basic operational principles of a liquid section which utilises athermal actuator device will be explained with reference to FIG. 1. Itis to be understood that the thermal actuator of the invention is notlimited to use in such liquid ejection devices. As shown in FIG. 1,there is provided an ink ejection arrangement 1 which comprises a nozzlechamber 2 which is normally filled with ink so as to form a meniscus 10around an ink ejection nozzle 11 having a raised rim. The ink within thenozzle chamber 2 is resupplied by means of ink supply channel 3.

The ink is ejected from a nozzle chamber 2 by means of a thermalactuator 6 which is rigidly interconnected to a nozzle paddle 5. Thethermal actuator 6 comprises two arms 8, 9 with the bottom arm 9 beinginterconnected to an electrical current source so as to provideconductive heating of the bottom arm 9. When it is desired to eject adrop from the nozzle chamber 2, the bottom arm 9 is heated so as tocause rapid expansion of this arm 9 relative to the top arm 8. The rapidexpansion in turn causes a rapid upward movement of the paddle 5 withinthe nozzle chamber 2. This initial movement causes a substantialincrease in pressure within the nozzle chamber 2 which in turn causesink to flow out of the nozzle 11 causing the meniscus 10 to bulge.Subsequently, the current to the heater 9 is turned off so as to causethe paddle 5 to begin to return to its original position. This resultsin a substantial decrease in the pressure within the nozzle chamber 2.The forward momentum of the ink outside the nozzle rim 11 results in anecking and breaking of the meniscus so as to form a meniscus and abubble of ink 18. The bubble 18 continues forward onto the ink printmedium as the paddle returns toward its rest state. The meniscus thenreturns to the position shown in FIG. 1, drawing ink past the paddle 5in to the chamber 2. The wall of the chamber 2 forms an aperture inwhich the paddle 5 sits with a small gap there between.

FIGS. 2 and 3 show a first embodiment of a thermal bend actuatoraccording to the invention.

The actuator has two lower arms, 27, 28 and three upper arms, 23, 25 and26. All of the arms are formed as a unitary structure and are joined byan integral cross arm 32.

As best seen in FIG. 3, the edges of the upper and lower arms arelocated in a common plane 33. However, the lower arms 27, 28 areconfigured so as to extend below the common plane 33 whilst upper arms23, 25, 26 extend above the common plane. The result is that thelongitudinal centre of inertia for the upper arms is above the plane 33and the longitudinal centre of inertia for the lower arms is below theplane, as indicated by lines 34 and 35 respectively.

The free ends 36, 37 of the lower arms are selectively connected to anelectrical power source which causes current to pass from arm 27, intocross arm 32 and then into arm 28 (or vice versa), causing resistiveheating of arms 27 and 28. The free ends 38, 39, 40 of arms 23, 25, 26respectively are not connected to the electrical circuit so current doesnot flow into them. Thus, the arms 23, 25, 26 are not heated and do notexpand. Ends 36, 37, 38, 39 and 40 are secured to an anchor block (7 inFIG. 1) so the thermal expansion in the length of the lower arms 27 and28 results In an upward bending of the actuator as a whole about theanchor block. This, in turn, causes movement of whatever device, such asan ink ejection paddle, is connected to the actuator.

In cross-section outer arms 23 and 26 have a central horizontal portion41 and two downwardly extending portions 42, which extend symmetricallydownwards to the common plane 33.

The central arm 25 has, in cross section, two outer portions 44 and twoinner portions 45. The two inner portions 45 extend symmetricallyoutwards and upwards relative to each other and the centre line of thearm 25. The outer portions 44 then extend downwardly and outwardlyrelative to the two inner portions to extend to the common plane 33.

The two lower arms 27 and 28 are identical and include a central portion46 located in the common plane 33, two intermediate portions 47extending downwardly from the central portion 46 and then two outerportions 48 extending upwardly to the common plane 33.

The two lower arms 27 and 28 are spaced equally between the centralupper arm 25 and the outer arms 26 and 23 respectively so that arm 25lies in the centre of the actuator as a whole. The total cross sectionalarea of the three upper arms 23, 25 and 26 is equal to the total crosssectional area of the two lower arms 27 and 28, but this is notessential. It will be appreciated that the non planar arms have agreater stiffness and hence resistance to bending compared to arms ofthe same cross sectional area.

FIG. 4 shows a cross-section of a second embodiment in which there areprovided three upper arms and two lower arms. In plan view theconfiguration of the arms is the substantially the same as that shown inFIG. 2 for the first embodiment. The embodiment of FIG. 4 only differsin the cross sectional profile of the upper and lower arms.

The outer upper arms 80 and 81 have the same profile as arms 23 and 26of the first embodiment. The central upper arm 82 is also similar to theouter arms 23 and 26 of the first embodiment in that it has a centralportion with two outwardly and downwardly extending portions whichterminate at the common plane 33. It will be noted that the width of thecentral portion 85 of arm 82 is greater than that of the two outer arms80 and 81, but this is not essential and all three upper arms may beidentical in cross section, if desired.

The two lower arms 83 and 84 are similar to those of the firstembodiment but lack a central portion. Instead, outer portions 86 extendinwardly and downwardly from the common plane 33 to meet with innerportions 87 which extend outwardly and downwardly from the common plane33 to form a W shape.

The arrangement of actuator arm cross-sections, as shown in FIGS. 2 and3, provides for an increased stiffness in the thermal bend actuatorwithout increasing the thickness of the layer through the corrugatednature of the actuator arms and reduces the occurrence of bucklingduring operation of the thermal bend actuator compared to an actuatorwith planar arms of the same total cross sectional area. This allows thecross sectional area, and hence mass, to be reduced, so increasingefficiency. In addition this allows stiffness to be increased to reducethe risk of buckling, with or without a reduction in mass.

As all the edges of the upper and lower arms are in the same plane, thisprovides for an increase in the exposure precision and allows either theactuator arms to be brought closer together than would be otherwisepossible or to utilise equipment having a smaller depth of field (modernstepper equipment often operates over a depth of field of approximately0.5 microns). In addition, as will be explained below, this also allows,in preferred embodiments for a simplified manufacturing process.

Whilst the total cross-sectional areas of the upper and lower arms inthe two embodiments are similar, it is to be appreciated that the exactshape, cross section of each arm or total cross sectional area is notcritical so long as the upper arms as a whole bend about an axisvertically distant from the axis about which the lower arms bend.Further whilst the edges of the arms preferably lie in a common plane,this is not essential.

In both of the embodiments described the effective moment of inertiaand/or mass of the upper arms is preferably equal to that of the lowerarms for optimum efficiency. However, this is not essential and theeffective moment of inertia and/or mass of the two sets of arms may bedifferent. In addition the centres of inertia of the two sets of armsmay be at unequal distances from the common plane 33.

It is also to be appreciated that the invention is not limited to thenumber of upper arms being in a ratio of 3:2 to the number of lowerarms. For example, without limiting the scope of the invention, ratiosof 2:2, 5:3, 4:4 are acceptable.

Manufacture of the thermal band actuator according to the invention willbe described with reference to FIGS. 5 and 7 which shows cross sectionsof a partially fabricated ink ejection device utilising a thermal bandactuator according to the first embodiment. FIG. 6 also shows apartially fabricated thermal bend actuator according to the secondembodiment of the invention. The process steps of manufacture of thefirst and second embodiments are the same and it is only the design ofthe masks used that differentiate the end product.

Referring to FIG. 5, the manufacturing steps for the formation of thethermal bend actuator include the following steps:

-   1. A CMOS wafer 20 is provided having the required electrical    control structures formed thereon;-   2. A first sacrificial layer 22 is deposited and etched to form a    generally planar spacer layer. The layer 22 is preferably deposited    as a 2 microns thick layer of photoimageable polyamide (PI) by    spinning on the layer. The layer is then exposed through a first    mask, developed and etched so as to remove unexposed regions and    then hard-baked. During the hard-bake stage, the PI shrinks to 1    micron thickness.-   3. A second sacrificial layer 21 is deposited on selected areas of    the first layer in the same manner as step 2, but utilising a    different mask. The second layer 21 does not extend over all of the    first layer but is laid so as to have gaps 19 between portions. On    baking the PI layer shrinks and, as is its nature, it forms side    walls which are angled at about 45□ to the general horizontal plane    on either side of the gaps 19. In the drawings the gaps 19 are shown    as having minimal width at the interface with the first layer 22.    However it is to be appreciated that the gaps 19 may have a    significant width at the interface with the first layer 22;-   4. A third sacrificial layer 23 is then deposited onto selected    areas of the second layer 21 to form three arms. Again the edges    become angled upon baking in the same manner as in step 2;-   5. A single 0.25 micron layer of titanium nitride 18 is then    deposited to cover all of the exposed first, second and third    sacrificial layers 21, 22, 23. The titanium nitride layer is then    etched by known means to remove material between the intended arms    and to form gaps between the arms and pattern the layer 18 so as to    form a thermal bend actuator having the cross-section of either FIG.    3 or FIG. 4, depending upon the mask pattern design in previous    steps;-   6. A fourth sacrificial layer 59 of 6 micron thickness is then    deposited so as to form a sacrificial structure for a nozzle    chamber;-   7. A 1 micron layer of dielectric material 61 forming the nozzle    chamber 2, is deposited;-   8. Subsequently, a 1 micron layer of sacrificial material comprising    the fifth sacrificial layer (not shown) is deposited;-   9. A subsequent layer (not shown) is also deposited to form a nozzle    rim, with the etch preferably being an anisotropic etch so as to    leave a thin nozzle rim structure around the nozzle chamber;-   10. A nozzle guard structure (not shown), if required, is attached    to the substrate. The nozzle guard structure can be independently    micromachined to mate with the substrate;-   11. The nozzle guard is then affixed to UV-sensitive adhesive tape;-   12. The CMOS wafer is then back etched utilising a Bosch etching    process to separate the wafer into printhead segments and to form an    ink channel supply channel to the nozzle chamber; and-   13. the UV tape is then exposed to light so as to reduce its    adhesiveness and to leave individual printhead segments which can be    picked and placed for testing.

In the above process the titanium nitride layer for both sets of arms islaid down in one step. This results in a reduction in the process stepscompared to depositing material for the two sets of arms sequentially.Further more the spacing of edges of adjacent arms is only limited bythe accuracy of the stepper used.

The above steps simultaneously form both the thermal bend actuator andthe nozzle chamber and paddle. For example, in FIG. 7, there is shown aportion 50 of the nozzle chamber (which is symmetrical around an axis52). On the wafer 20 is formed the CMOS layer 55. The first sacrificiallayer 54 is deposited and etched as are the second 57 and third 58sacrificial layers. On top of the sacrificial layers is deposited atitanium nitride layer 60, which includes portions forming the paddle aswell as the actuator. A subsequent fourth sacrificial layer 59 isdeposited and etched. This is followed by a dielectric layer 61 which isdeposited and etched so as to form the nozzle chamber proper.Subsequently, the sacrificial layers are etched away so as to releasethe bend actuator and simultaneously release the paddle structure.

It would be appreciated by a person skilled in the art that numerousvariations and/or modifications may be made to the present invention asshown in the specific embodiment without departing from the spirit orscope of the invention as broadly described. The present embodiment is,therefore, to be considered in all respects to be illustrative and notrestrictive.

1. A thermal actuator for micro mechanical or micro electro-mechanical devices, the actuator including: a) a supporting substrate, b) an actuator extension portion, c) an anchor block extending from the substrate d) a first group of at least two elongate corrugated arms attached at a first end thereof to a heating circuit at the anchor block and at a second end to the extension portion, the first group being arranged, in use, to be conductively heated through connection to the heating circuit, e) a second group of at least two elongate corrugated arms attached at a first end to the anchor block and at a second end to the extension portion, the second group not being connected to a heating circuit and being spaced apart from the first group,  wherein the corrugations of the corrugated arms of the first group project from the plane of the extension portion in a direction opposite to a direction in which the corrugations of the corrugated arms of the second group project, and the first group is arranged, in use, to undergo thermal expansion, thereby causing the actuator to apply a force out of said plane to the extension portion.
 2. An actuator of claim 1 wherein the arms of said first group and said second group are spaced transversely relative to each other and, in plan view, do not overlap.
 3. An actuator of claim 1 wherein, in transverse cross-section, an arm of said first group or an arm of said second group includes an edge portion extending away from the other of the second and first arm.
 4. The actuator of claim 3 wherein, in transverse cross-section, the edge portion extends inwardly.
 5. The actuator of claim 1, wherein an arm of said first group and/or an arm of said second group, has, in transverse cross-section, a U, V, C or W profile.
 6. An actuator of claim 1 wherein said second group includes three elongate arms.
 7. An actuator of claim 1 wherein the first group and the second group are formed as an integral unit, said first and second groups being joined at the second end to a transversely extending cross arm. 